1. Field of the Invention
The present invention relates to a biosensor, and more particularly to a biosensor having a chip with measuring reliability and a method for improving measuring reliability thereof.
2. Description of the Prior Art
In recent years, various kinds of biosensors utilizing a specific catalytic action of enzymes have been developed to be used for clinical purposes. Most valuable use of such biosensors may be made in the area of e.g. diabetes treatment where it is vital for patients to keep their blood glucose concentration (“blood sugar level” below) within a normal range. For an inpatient, the blood sugar level can be kept normal under the supervision of the doctor. For an outpatient, self-control of the blood sugar level is an important factor for treatment in lack of doctor's direct supervision.
The self-control of the blood sugar level is achieved through a diet, exercise and medication. These treatments may often be simultaneously employed under the supervision of the doctor. It has been found that the self-control works more effectively when the patient himself is able to check whether or not his blood sugar level is within the normal range.
Recently, blood sugar determining instruments have been used for self-checking of blood sugar level. As shown in FIG. 1, a blood sugar determining instrument mainly includes a main detecting unit 10 and a chip 12 for blood sugar measurement. As shown in FIG. 2, the chip 12 includes a strip-like substrate 122 provided in its front portion with an electrode section 1221. The electrode section 1221 is covered by a reaction layer 124, a spacer 126 and a cover sheet 128. The electrode section 1221 is provided with an operational terminal 1222 and a counterpart terminal 1224 surrounding the operational terminal 1222. The operational terminal 1222 and the counterpart terminal 1224 are electrically connected to lead terminals 1226 and 1228, respectively, which are formed on a base end portion of the strip-like substrate 122. The reaction layer 124, which covers the electrode section 1221, contains potassium ferricyanide and an oxidase such as glucose oxidase.
The blood sugar determining instruments may be used in the following manner. A patient pricks hi or her own skin with e.g. a lancet for oozing blood. Then, the oozed-out blood is caused to touch the tip of the chip 12 plugged into the main detecting unit 10. The blood is partially sucked into the reaction layer 124 by capillary action. The reaction layer 124 disposed above the electrode section 1221, is dissolved by the blood, which starts an enzyme reaction, as the following formula:
      D    ⁢          -        ⁢    Glucose    +      2    ⁢                  Fe        ⁡                  (          CN          )                    6              3        -              +            H      2        ⁢          O      ⁢                                        ⁢                                      ⁢              ⟶        GOD            ⁢                          ⁢      Gluconic        ⁢                  ⁢    acid    +          ⁢      2    ⁢          H      +        +      2    ⁢                            Fe          ⁡                      (            CN            )                          6                  4          -                    .                          ⁢                                    Fe            ⁡                          (              CN              )                                6                      4            -                          ⁢                                  ⁢                  ⟶          Voltage                ⁢                                  ⁢                              Fe            ⁡                          (              CN              )                                6                      3            -                                +      e    -  
Potassium ferrocyanide is produced in an amount corresponding to the glucose concentration. After a certain period of time, a predetermined voltage Vref is applied on the chip 12 to electrochemically oxidize potassium ferrocyanide to release electrons. A response current is generated and passes through the operational terminal 1222. The response current is proportional to the concentration of potassium ferrocyanide produced by the enzyme reaction or to the concentration of the glucose. Therefore, the blood sugar level can be known by measuring the response current.
FIG. 3 is a schematic diagram of a control circuit of the blood sugar determining instrument of FIG. 1, in which the electrode section 1221 of the chip 12 can be regarded as a resistor Rs. The voltage Vref to be applied can be provided by a battery. The response current I generated by the chip 12 is converted to an output voltage Vout by a current/voltage converter 30 having an amplification resistance Rf. The output voltage Vout is represented by the formula (I):Vout=(1+Rf/Rs)Vref  (I),
A microprocessor (not shown) processes the output voltage Vout through the analog to digital converter (not shown), and accordingly calculates the glucose concentration of the blood sample. A reading of the glucose concentration is displayed on a display such as a liquid crystal display (LCD) (not shown).
However, the resistance difference among the substrates 122 (or the chips 12) is caused during the manufacturing process thereof. In general, the resistance of the resistor Rs of the substrate 122 is in the range of 2K to 4K. Referring to the formula (I), due to the resistance difference of the substrates 122, the output voltage Vout caused by each chip 12 in response to the same blood sample is different. As a result, different chips 12 monitor different glucose concentrations for the same blood sample. Therefore, the resistance difference of the resistor Rs of the substrate 122 reduces the measuring reliability of the chip 12.
Accordingly, it is an intention to provide means for improving measuring reliability of the chip, which can resolve the problem encountered by the conventional biosensor.